Vertex Paint not painting the areas selected on Blender2.8
This has an answer here.

Is it possible to paint only on selected vertices in vertex paint?

Though I would like to add that if you are in any viewport shading mode other than Solid, the results won't show up in the 3D viewport when you are painting in vertex paint mode.

Also of note that after you get this working, you're very likely going to want to perform a render at some point. And you may run into more trouble when you get to that point, because vertex paint doesn't seem to want to render unless you go into the Shader Editor and hook up some nodes. If you later find that this is a problem, consider reviewing the following:

How can vertex paint be rendered?

How do I set a vertex color material in Blender 2.8?